1. Field of the Invention (Technical Field)
Embodiments of the present invention relate to acoustic cytometry and more specifically to acoustic focusing hardware and implementations.
2. Background
Note that the following discussion refers to a number of flow cytometry systems. Discussion of such systems herein is given for more complete background and is not to be construed as an admission that such systems are prior art for patentability determination purposes.
Flow cytometry is a powerful tool used for analysis of particles and cells in a myriad of applications primarily in bioscience research and medicine. The analytical strength of the technique lies in its ability to parade single particles (including bioparticles such as cells, bacteria and viruses) through the focused spot of light sources, typically a laser or lasers, in rapid succession, at rates exceeding thousands of particles per second. The high photon flux at this focal spot produces scatter of light by a particle and/or emission of light from the particle or labels attached to the particle that can be collected and analyzed. This gives the user a wealth of information about individual particles that can be quickly parleyed into statistical information about populations of particles or cells.
In traditional flow cytometry, particles are flowed through the focused interrogation point where a laser directs a laser beam to a focused point that includes the core diameter within the channel. The sample fluid containing particles is hydrodynamically focused to a very small core diameter of around 10-50 microns by flowing sheath fluid around the sample stream at a very high volumetric rate on the order of 100-1000 times the volumetric rate of the sample. This results in very fast linear velocities for the focused particles on the order of meters per second. This in turn means that each particle spends a very limited time in the excitation spot, often only 1-10 microseconds. When the linear flow of the hydrodynamic sheath fluid is stopped, the particles are no longer focused. Only resuming the hydrodynamic sheath fluid flow will refocus the particles. Further, once the particle passes the interrogation point the particle cannot be redirected to the interrogation point again because the linear flow velocity cannot be reversed. Still further, a particle cannot be held at the interrogation point for a user defined period of time for further interrogation because focusing is lost without the flow of the hydrodynamic sheath fluid. Because of the very high photon flux at the excitation point, flow cytometry is still a very sensitive technique, but this fast transit time limits the sensitivity and resolution that can be achieved. Often, greater laser power is used to increase the photon flux in an effort to extract more signal but this approach is limiting in that too much light can often photobleach (or excite to non-radiative states) the fluorophores being used to generate the signal and can increase background Rayleigh scatter, Raman scatter and fluorescence as well.
Slower flowing cytometry systems have been developed to push the limits of sensitivity and have shown detection limits down to the single molecule level. In one of these systems, it was shown that lower laser power (<1 mW) was actually preferable for single molecule detection of double stranded DNA fragments intercalated with fluorescent dyes. Because of the slow transit times (hundreds of microseconds to milliseconds), it was possible to get maximum fluorescence yield out of the dyes while reducing background, photobleaching and non-radiative triplet states with the lower laser power.
Slow flow hydrodynamic systems, while incredibly sensitive, are not in widespread use because fluidic dimensions are generally very small, which results in easy clogging and very limited sample throughput. In order to focus the sample stream to a core diameter small enough to maintain the uniform illumination and flow velocity required for precision particle measurement, the sheath must still be supplied in a very high volumetric ratio to the sample. In order to achieve a slow linear velocity, the volumetric sample rate must be extremely small. Therefore, to process appreciable numbers of events, the sample must be highly concentrated. If for example a relatively slow linear velocity of 1 centimeter per second is desired with a typical core diameter of about 10 microns, the sample must be delivered at about 0.05 microliters per minute. To process just 100 cells per second, the cell concentration must be 120,000 per microliter or 120 million per milliliter. This concentration requirement in turn makes clogging even more likely. The problem is further compounded by the tendency of many types of cells to clump in high concentration and to settle out and stick to surfaces when sample delivery rates are slow. The system created by Doornbos, circumvents the clogging problem by using a conventional flow cell with flow resistors to slow the flow, but he found it very difficult to control precise focused delivery of the sample. This method also does not eliminate the need for slow volumetric delivery and highly concentrated samples.
Sheathless, non-focusing flow cytometers have been developed but these instruments suffer from low sensitivity due to the need for a focal spot size that will excite particles throughout the channel. The spot size is reduced by using very small capillary channels but particles flow within the channel at variable rates according to the laminar flow profile that develops in the channel. This results in different transit times and coincidence of particles in the laser spot which both make analysis more difficult. Also, background cannot be reduced by spatially filtering optics that are designed to collect light from a tightly focused core stream. This limits sensitivity and resolution.
Other approaches have been demonstrated to manipulate particles using acoustic radiation pressure in a laboratory setting. These devices are planar devices modeled in Cartesian coordinates. Applying an acoustic field generates a quasi-one-dimensional force field that focuses particles into a ribbon in a rectangular chamber. For laminar flow, the resulting distribution of particles across the chamber places the particles in different velocity stream lines. Particles in different stream lines will not only be in different locations but they will also flow at different velocities. This in turn results in different residence times for particles at a location within the device. Planar focusing does not align particles in a manner suitable for use with flow cytometers.